Use of an expansion chamber for the production of representative particulate effluent from polymers

ABSTRACT

A gas stream monitoring system monitors a gas stream of a power generator and causes a sample of the gas stream to be collected if its characteristics indicate that a material in the power generator is being thermally degraded. After verifying that the degradation indication is true, a sample is taken of the gas stream. In order to insure that the sample is representative, an expansion chamber approximately the same size as the ionization chamber of the monitoring device is placed in line prior to the sampling device. The sampling device has three sections which collect large particles, small particulates, and vapors and gases. The products collected can be analyzed to determine which material in the power generator was thermally degraded.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to gas stream monitoring apparatus andparticularly to such apparatus for use with gas cooled dynamoelectricmachines.

2. Description of the Prior Art

Large dynamoelectric machines occasionally fail due to thermaldegradation of various materials, particularly organic insulation. Sincean early detection of the insulation failure is essential to theprevention of a large scale burnout of the dynamoelectric machine,monitoring devices are desirably used which monitor the gas streams thatflow through dynamoelectric machines. Presently, most monitors work bydetecting particulates in the gas stream, which are found wheninsulation is being thermally degraded. When the monitor detectsdegradation products and generates a signal, the flow of the detectableparticulates to the monitor is terminated to determine whether thesignal is genuine or is due to a malfunction of the monitor. If thesignal then terminates it is assumed to be genuine and the generator canbe shut down for repair or other precautions taken.

Since down time on a large generator can be costly, it is normallyimportant to locate the insulation failure and repair it quickly. Over50 different materials are used in generators, including regular andmodified expoxies, polyesters, silicones, phenolics, etc., and unlessthe failure is easily visible, it may be very difficult to locate.

In U.S. Pat. No. 3,972,225, issued Aug. 3, 1976, it was disclosed thatif the gas stream is sampled when the monitor indicates that a failureis occurring, the products collected can be analyzed to determine whichmaterial in the generator was failing. Since the location of the variousmaterial is known, the search for the failure is considerably shortened.

It was also disclosed that the sampling can be done automatically, sothat when the monitor produces a signal it can check for authenticityand the sample taken without human interference.

Finally it was disclosed that a particular sampling device, whichseparates the products of the gas stream into particles 10 microns orgreater, particulates less than 10 microns, and vapors and gases, isparticularly useful in facilitating the analysis.

SUMMARY OF THE INVENTION

We have found that one of the features of the generator conditionmonitor used in the prior art is that the ionization chamber containingan alpha source is also a mixing chamber. This chamber is, for example,approximately 7.6 cm in diameter by approximately 25.4 cm long producinga volume of about 1150 cc. This volume is very large in comparison tothe pipe feeding it. Thus, it will act as a gas expansion or coolingchamber. In the case where a condensation nuclei type of monitor isused, an expansion chamber is an integral part of its operation.

We have found that by using the prior art system as disclosed in theabove-referenced patent and inserting in that system prior to thesampling chamber an expansion chamber, that the resulting cooling andincrease in formation residence time aids significantly in theproduction of aerosol condensates to which the generator conditionmonitor had responded.

While of particular interest for use with gas (e.g., hydrogen)-cooledelectrical apparatus (e.g., generators), it will be apparent theapparatus of this invention may be applied to other gas systems.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the invention, references may be had tothe preferred embodiments, exemplary of the invention, shown in theaccompanying drawings in which:

FIG. 1 is a diagram showing a presently preferred sampling system withan expansion chamber according to this invention;

FIG. 2 is a schematic view of the ionization chamber of the generatorcondition monitor of FIG. 1;

FIG. 3A is a mass spectra of particulates collected by the prior artmethod;

FIG. 3B is a mass spectra of particulates collected using the systemincluding an expansion chamber in accordance with this invention.

FIG. 4 is a schematic view of a condensation nuclei dielectric which canbe used as an alternate for the detector shown in FIG. 2;

DESCRIPTION OF THE PREFERRED EMBODIMENT

In FIG. 1, conduit 1 carries gas from the gas stream of thedynamoelectric machine 2 to monitor 3 where it is monitored and thenreturned to the machine through conduit 4.

Monitor 3 checks the gas stream for any characteristics which indicatesthat thermal degradation of material in the generator is occuring. Thethermal degradation may be due to primary failure of the material, ormay be a secondary failure which indicates arcing or some other problemis occurring in the generator. Typically, the monitor checks the levelof particulates in the gas stream. The operation of the inoizationmonitor as shown in FIG. 2 is more particulary described in U.S. Pat.No. 3,573,460, issued Apr. 6, 1971. However, if the monitor is thesimplified condensation nuclei type of monitor as shown in FIG. 4, thena more detailed description is provided in U.S. Pat. No. 3,427,880,issued Feb. 18, 1969.

Particulates which are small particles or condensed gases, (i.e.,liquids, aerosols) less than about 10 microns in size, are formed whenorganic materials thermally degrade. In fact, some of the materials inthe generators may be coated with substances which produce largequantities of particulates at specific low temperatures to aid in theearly detection of insulation overheating before failure. Other types ofmonitors may be used to detect the failure of materials by checking forvapors in the gas stream, pH of the gas stream, optical density oroptical absorption of the gas stream, or other characteristics orcombination of characteristics.

Referring to FIG. 1, when the monitor detects a failure of a material,it generates a signal, typically an electric signal, which passesthrough line 5 to sampling controller 7.

Insulation occasionally falls off or is abraded off by friction and someof its dust can be made airborne by the moving parts of the generator.If these dust particles are present in sufficient quantities, they cancause monitor 3 to produce spurious signals for brief periods. Thesesignals do not indicate a genuine material failure due to heat. Samplingcontroller 7 therefore will block out the flow of gas to the monitor bypassing a signal through line 8 and closing valve means 9.

Since stopping a dynamoelectric machine and searching for insulationfailure is a very expensive undertaking, monitor 3 must first be checkedto determine that it is operating correctly. Depending on the type ofmonitor used, solenoid valve means 9 therefore terminates the flow ofgas to the monitor or diverts the gas through a filter (not shown) whichfilters out of the gas streams, the particulates or gases that activatedthe monitor.

If the signal from the monitor then terminates, sampling controller 7will generate a signal which passes through line 10 to solenoid valve 12which controls the flow of the gas stream to the expansion chamber 6 andsampling device 13, then to the exhaust system (not shown) throughconduit 14.

Referring now to FIG. 2, the generator condition monitor 3 preferablycomprises an ionization chamber 52 and an ion collection chamber 54contained in a pressure housing 56. The gas stream flow as representedby arrows 58 passes through the ionization chamber 52 in which a lowlevel radiation source 60 is disposed. The convenient means for ionizinghydrogen gas, which is generally the cooling gas used in dynamoelectricmachines comprises a minute amount of thorium 232, which produces 3.999Mev alphas, and has a half-life of 1.32 × 10¹⁰ years.

The ionization chamber is approximately 7.6 cm. in diameter by 25.4 cm.long and has a volume of approximately 1150 cc. This volume is verylarge in comparison to the pipe feeding it. Therefore, it acts as a gasexpansion chamber by cooling the gas as it flows into the chamber. Thiscooling, by increasing the formation residence time, aids in theproduction of aerosol-type condensates to which the condition monitorresponds.

The cooling gas ions produced by the thorium source 60 are carried bythe coolant flow into the ion collection chamber 54 which has a pair ofcollection electrodes (not shown).

If the gas stream monitor is the simplified condensation nuclei detectorshown in FIG. 4, gas from the generator enters the detector throughconduit 1 as shown in FIG. 1, is reduced in pressure by reducing valve30 and diluted with an inert gas, such as nitrogen from bottle 31. Thediluted mixture passes through a humidifying chamber 32 which saturatesthe gas with water vapor. An expansion chamber 33 serves to cyclicallyexpand batches of humidified gas when valves 34 and 35 are alternatelyopened and closed by an automatic rotary valve actuator 36. A lightsource 37 and lens system 38 causes the scattered light to be detectedby photo tube 39. When the adiabatic expansion occurs, the water vaporcondenses on the condensation nuclei (the particulates if present). Thescattered light intensity falling on photo tube 39 is related to thenumber of droplets affected by the presence of particulates. The lightresponsive signal passes through line 5 to sampling controller 7. Incontrast to the system in FIG. 1, the gas after passing through thecondensation nuclei detector is preferably discarded through conduit 104which is connected to an exhaust system (not shown).

Regardless of the type of monitors used, it is felt, in order to collecta more representative sample of the particulate to which the monitor hadresponded, that the gas, with particulates therein, must be providedwith an adequate amount of formation residence time such as thatprovided by the above discussed detector before being collected bysampling.

Therefore, we have found by laboratory means that if an expansionchamber was inserted in a gas stream in front of the sampling device,the results of the mass spectra of the collected sample is quitedifferent from the resulting mass spectra of the collected samplewithout the expansion chamber. In the laboratory experiment a sample ofp-Toluenesulfonic acid in SC 193/1 (a modified epoxy coating system) wasplaced in a tube furnace and hydrogen gas was flowed through the furnacewhile the furnace was being heated. There was a standard ionization typegenerator condition monitor in line with the tube furnace and upon anindication that there were thermoparticulates being produced, thegenerator condition monitor was removed from the line and the flow ofhydrogen through the tube furnace was diverted to an effluent trap whichcollected particulates and vapors. After a sample was collected the massspectra was ran on the collected samples and the results are shown inFIG. 3A.

Using the same laboratory setup a generally tubular shaped expansionchamber of 945 cc. was inserted in line between the tube furnace and theeffluent trap. Once again, the furnace was heated and as a flow ofhydrogen gas passed through it, a second sample was collected. Afterthis sample was collected a mass spectra of the particulates collectedin the effluent trap was performed and the results are shown in FIG. 3B.In comparison of the two figures, the mass to charge ratio under theprior art system shows that there were no aerosol condensates orparticulates present with a mass to charge ratio greater than 60.However, with the addition of the expansion chamber there are manysignificant peaks with a mass to charge ratio greater than 60. Amongstthese are at 94, at 120 and at 136.

The present particulate traps do not have any ante-chamber which can actas an expansion and cooling compartment. Furthermore, at a typical gassampling rate of 5 liters per minute, a particle will have approximatelya 14-second formation residence time in a prior art generator conditionmonitor ionization chamber. By contrast, the standard particulatecollector trap has dead space preceeding the particulate collector discthat permits a formation residence time for a particle of only about0.03 seconds at a sample flow rate of 5 liters per minute.

Therefore, in order to collect a more representative sample ofparticulates, that the monitor responded to, an expansion chamber shouldbe inserted in line prior to the sampling device that will increase theformation time of the particulates beyond the current prior art system's0.03 seconds at a flow rate of approximately 5 liters per minute ofhydrogen.

We claim:
 1. A sampling system comprising:a gas stream monitoring meansfor detecting thermal degradation of materials by monitoring the gasstream exposed to the products of said thermal degradation, and forgenerating a first signal when said thermal degradation is detected;means for determining whether said gas stream monitoring means isfunctioning properly when said first signal is generated and if so, forgenerating a second signal; a sampling means for sampling said gasstream when said second signal is generated; and a gas expansion meansfor increased production of aerosol-type condensates disposed betweensaid sampling means and said gas stream.
 2. The sampling systemaccording to claim 1 wherein said gas expansion means further comprisesa generally tubular shaped chamber disposed in a connecting conduitbetween said gas stream and said sampling means, said generally tubularshaped chamber having an internal dimension substantially larger thansaid connecting conduit's internal dimension.
 3. In combination adynamoelectric machine having a gas stream, a gas stream monitor havingan ionization chamber and an ion collection chamber, a sampling devicefor sampling said gas stream and a generally tubular shaped expansionchamber for production of aerosol-type condensates, said expansionchamber connected by inlet conduit means to allow gas streamcommunication thereto from said monitor, said expansion chamber having avolume large enough to facilitate the cooling of said gas stream therebyproducing aerosol condensates, and conduit means to allow communicationbetween said expansion chamber and said sampling device.
 4. Thecombination according to claim 3 wherein said expansion chamber has aninternal dimension and a volume equal to said ionization chamber.
 5. Thecombination according to claim 3 wherein said expansion chamber'sinternal dimensions and volume provide a formation residence timegreater than about 0.03 seconds at a flow rate of about 5 liters perminute of said gas stream.